Evolving phytoplankton primary productivity patterns in typical Tibetan Plateau lake systems and associated driving mechanisms since the 2000s

Phytoplankton primary productivity from the surface to the euphotic depth (PPeu) is an important indicator for estimating the carbon (C) sequestration capacity of a lake system. Approximately 50% of China's lake systems are distributed throughout the Tibetan Plateau (TP), playing an important C neutrality role in China and beyond. Over the past 20 years, the PPeu of TP lake systems may undergo dramatic spatial and temporal transformation under the significant influence of climate change and human activities. To reveal corresponding changing mechanisms, this study explored spatiotemporal PPeu patterns in typical TP lake systems with MODIS observations during 2002–2020. Since the 2000s, results reveal that the annual mean PPeu in TP lake systems ranges from 476.7 to 621.6 mg C/m2/d, with an average value of 553 ± 36.2 mg C/m2/d. Spatially, PPeu was much higher on the northeastern boundary and in the south of the TP compared to its remaining northern regions. PPeu trends in TP lake systems with higher salinity levels and those at higher altitudes were lower. Temporally, 48% of lakes on the TP exhibited a significant descending PPeu trend, while 31% of lakes exhibited a significant ascending trend. PPeu in TP lake systems may have mostly been influenced by day length (decimal hours), land surface temperature, and chlorophyll-a (Chl-a) content while being mainly limited by relative humidity (RHU). Additionally, PPeu also showed a significant positive relationship with total N and P imported from wastewater in TP (especially in Tibet), indicating that anthropogenic emissions could have an important influence on PPeu. Spatiotemporal pattern and driving mechanism results from this study can help us better understand the C sequestration capacity of TP lake systems.